Ghodssi R., Lin P., MEMS Materials and Processes Handbook

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374 T. Mineta and Y. Haga

Heat flow

Temperature

Heating

Cooling

Heating

Strain

Cooling

Exo.Endo.

Fig. 6.17 Typical strain and

DSCcurvesofTiNiSMA

with the R-phase

transformation

have been studied in many reports on bulk materials [2] and thin ﬁlms [4, 52]. The

addition of Cu is effective to decrease thermal hysteresis, which is the difference of

phase transformation temperatures between the heating and cooling processes [53].

Small thermal hysteresis is favorable for repeated actuation in a narrow working

temperature range between heating and cooling. TiNiCu bulk alloys containing 5–

15 at.% Cu transform in two stages, from a cubic to an orthorhombic B19 phase, and

then to a monoclinic B19’ phase [2]. In the case of sputtered TiNiCu thin ﬁlm, the

alloy ﬁlm containing 0–9.5 at.% Cu transforms in a single stage, from B2 to B19’

[52]. In this region of single stage transformation, the width of thermal hysteresis

of the TiNiCu thin ﬁlm drastically decreases to 11 K with increasing Cu content.

TiNiCu ﬁlm containing 9.5 at.% Cu shows two-stage phase transformation, from B2

to B19, and then to B19’. By addition of more Cu, phase transformation becomes a

single stage from B2 to B19. When Cu content is below 9.5%, a maximum reversible

strain of 3.9% is obtained at a stress of 200 MPa. When Cu content is above 9.5%,

maximum reversible strain decreases to 1.1% with increasing Cu content to 18 at.%.

In many applications, TiNiCu alloys with Cu content from 5 to 10 at.% are used. The

addition of Cu also affects the mechanical properties, for example, the elastic limit

(yield stress) of martensite decreases in TiNiCu alloy. This behavior is favorable for

6 Materials and Processes in Shape Memory Alloy 375

actuator applications. TiNiCu SMA actuators can be reset by small external force in

the martensitic, resulting in large work generation [7].

The shape recovery temperature of SMA material is very important in many

applications. For example, SMA with a high deformation temperature is not suit-

able for medical devices for use in the human body due to thermal damage. The

deformation temperature of ternary alloy with added vanadium (TiNiV) tends to be

lower [54]. In contrast, a high deformation temperature is favorable for exact actua-

tion without deformation error caused by ambient temperature ﬂuctuation. Ternary

alloys with added palladium (TiNiPd) show a strong increase in transformation

temperature [6, 7].

TiNb Alloys (Ni Free)

TiNi alloys have been used extensively for permanent implants (stents and other

applications). Surface treatment is required to prevent corrosion. In recent years,

bio-compatible SMAs without nickel content such as NbTi-based alloy have also

been studied for application in medical devices for implantation in the human body

because the toxicity of nickel is viewed with suspicion [55].

6.2.6.2 Process

Common Mistakes

Thin Films

As mentioned in Section 6.2.2.1 (Sputtering), it is not easy to control the chemical

composition of deposited ﬁlm in the case of sputtering deposition using a single

alloy target. Film composition tends to differ from the composition of the target due

to the sputter rate difference between Ti and Ni. Sputtering with separate targets is

one solution to obtain good controllability of ﬁlm composition. However, special

equipment having multi-targets with individual electric power control is necessary.

To utilize the feature of l arge force generation, thick SMA ﬁlm is required in

many applications. However, it is not easy to obtain SMA ﬁlm thicker than 10

μm. To obtain thick SMA ﬁlm with sufﬁcient ﬁlm quality, i t is important to sta-

bly control ﬁlm composition. For example, in the sputtering process with an alloy

target, ﬁlm composition tends to change during thick ﬁlm deposition due to the

difference of sputter rate of Ti from Ni, as mentioned above. Figure 6.18 shows

examples of thick ﬁlm deposition by ﬂash-evaporation as mentioned in Section

6.2.2.2 (Evaporation) [23]. The cross sectional views of both TiNi (Ti-45 at.%Ni)

and TiNiCu (Ti-46 at.%Ni-0.5%Cu) thick ﬁlms show a lamellar structure, which

corresponds with the ﬂash-evaporation cycles. Top surface roughness of the TiNi

ﬁlm increases with increasing ﬁlm thickness due to the signiﬁcant growth of crys-

talline grains during the thick deposition. The grain growth results in defects in grain

boundary, leading to reduction of strength and ﬂexibility of the ﬁlm. In contrast, the

addition of a low concentration of Cu (<3 at.%) suppresses the grain growth of the

alloy ﬁlm. The TiNiCu ﬁlm is not fractured with a strain of 5% or larger, despite

being thicker than 30 μm.

376 T. Mineta and Y. Haga

18μm t

10μm t

20μm t

33μm t

Crack

22μ

m t

TiNi

TiNiCu

1μm

Fig. 6.18 Cross-sectional structures of ﬂash-evaporated thick SMA ﬁlms [23] (Reprinted with

Bulk

The material of most bulk SMA is TiNi alloy. Because of the poor mechanical work-

ability of TiNi alloy which results in high cost and defects in SMA, choice of shapes

of bulk SMA is limited.

Wires are fabricated by drawing and widely available. Coils are also commer-

cially available, but the cost of coiling is relatively high. Bulk plates of sheets

fabricated by rolling are relatively thick and not suitable for ﬁne patterning, for

example, micromachining or etching after photolithography. In the case of pipes or

tubes, although the cost of tube shaping is high, particular sizes of SMA tubes or

pipes are widely used for stents which are fabricated by laser cutting.

Micromachining

As a micromachining method for TiNi SMA, laser cutting is practical because it is

fast and allows more complex patterns [35]. To prevent deterioration due to heat gen-

eration by the laser, appropriate laser power and cutting speed should be selected. A

femtosecond laser is effective to prevent heat generation, but the cost of the system

is relatively high.

Etching

Oxide Film Removal

The surface of SMA materials tends to be oxidized during fabrication processes and

annealing for shape memorization at high temperature. When conducting the SMA

etching process, it is important to remove the oxide ﬁlm on the SMA surface before

etching because oxide ﬁlm prevents uniform etching of the SMA substrate. Oxide

6 Materials and Processes in Shape Memory Alloy 377

ﬁlm on the TiNi alloy surface is not dissolved by a mixed solution of ﬂuoric acid

and nitric acid. However, the oxide ﬁlm can be removed by lift-off as the result of

TiNi substrate etching. Using an ultrasonic cleaner improves the uniform removal of

the oxide ﬁlm in chemical etchant because the oxide ﬁlm can be removed promptly

when the TiNi alloy substrate beneath the oxide ﬁlm is etched. Oxide ﬁlm removal is

also important for complete electric contact with attached electrodes or lead wires.

Resist Selection for Chemical Etching

For patterning of TiNi alloys by chemical etching in a mixed solution of ﬂuoric acid

and nitric acid, an appropriate photoresist and baking process should be selected to

obtain sufﬁcient resistance to the etchant. Adhesion of photoresist to the SMA sur-

face is also important. Poor adhesion causes a large under-cut, which is undesirable

for accurate and ﬁne patterning.

Electrochemical Etching (Through-Layer Etching, Resist Selection)

For through-layer patterning of TiNi alloys by electrochemical etching, the back-

side of the SMA layer should be protected by a conductive material layer such as

Ni and Cu as mentioned in Section 6.2.4.1 to maintain uniform distribution of the

electrolytic current during over-etching. The dummy layers of Ni and Cu can be

removed selectively in concentrated nitric acid [40, 41].

Alcohol-based non-aqueous solutions such as H

-methanol, LiCl-methanol,

and LiCl-ethanol are useful as electrolyte solutions. Aqueous solutions are not suit-

able for electrochemical etching of TiNi alloys because the TiNi surface tends to be

anodically oxidized during etching. A photoresist with resistance to alcohol-based

solutions should be selected for the etching mask. Although many PMMA photore-

sists are solved in alcohol-based solutions, they are useful for the etching mask after

hard baking at high temperature.

Assembly

For assembly, mechanical and adhesive ﬁxations are simple and suitable for joining

SMA to dissimilar materials. The disadvantages of mechanical ﬁxation are a large

connection area, impreciseness of length adjustment, low productivity because of

the handmade process and susceptibility to fracturing in the ﬁxation area. A dis-

advantage of adhesive is that it is time consuming because of the application of

adhesive and the time required for curing.

Regarding the heating process for heat-curable adhesive, soldering or welding,

the temperature limit and heating time in the assembly should be considered to

prevent deterioration of the SME. As SMA deformed from its memorized shape

tends to be restored to its memorized shape during the heating process, a certain

ﬁxation method of the SMA is required to prevent the restoration.

Oxide ﬁlm on the surface of TiNi alloy should be removed to achieve uniformity

of electroplating as well as sufﬁcient solder wetting during soldering.

378 T. Mineta and Y. Haga

6.3 Applications and Devices

6.3.1 Medical

6.3.1.1 Stents

Treatment of stenosed (narrowed) lesions in the human body, for example, arterial

blood vessels, using dilatation of an inﬂatable balloon at the tip of the catheter is an

effective procedure called angioplasty.

Though the procedure is successfully performed, a high rate of restenosis occurs

after 6–12 months because of neointimal growth of smooth muscle cells in the arte-

rial blood vessels, formation of thrombi and recoiling of the vessel wall. Vascular

stents, which are metal tubes with mesh-like wall, are widely used for preventing

restenosis of treated blood vessels by mechanical expansion and scaffolding from

inside the blood vessel after angioplasty. However, they do not reduce neointimal

growth and there is still a risk of restenosis. Nonvascular stents are also used for

biliary, gastrointestinal, pulmonary, and urologic stenting.

Self-expanding SMA stents (Fig. 6.19) are effective for supporting the vessel

wall from inside the blood vessel and preventing migration of the stent [47]. Their

transformation temperature is typically set to 30

◦

C and the stent is superelastic at

body temperature. To improve biocompatibility, for example nickel r emoval from

the surface and corrosion resistance, electropolishing is occasionally performed. As

ﬂuoroscopic visibility of the TiNi is not sufﬁcient, radiopaque markers made of tita-

nium, gold or platinum-iridium are often attached to the end of the stent to facilitate

placement.

Fig. 6.19 Extreme

deformation of a TiNi stent

[47] (Reprinted with

Springer)

6.3.1.2 Endoscopes

Several ﬂexible endoscopes are used in the esophagus, stomach, duodenum (gas-

trointestinal endoscopy), colon (colonoscopy), and ureter and kidney (ureteropy-

eloscopy). These endoscopes can be bent (deﬂected) by manipulation of wires from

outside the human body.

6 Materials and Processes in Shape Memory Alloy 379

To pass through the colon, which meanders with a small radius, a steerable endo-

scope with a diameter of 13 mm and incorporating SMA coil actuators has been

developed. The SMA coil actuator contracts when heated by the application of an

electrical current from outside the body. Consequently, contraction of the SMA coil

bends the endoscope. The external diameter of the utilized SMA coil actuator is

1.0 mm and the diameter of its wire is 0.2 mm. Five bending segments are serially

connected and controlled synchronously with an external servomotor which moves

the endoscope back and forth from outside the body [56].

A steerable tip with a CMOS camera has also been developed using SMA coil

actuators. The external diameter of the utilized SMA coil actuator is 0.7 mm and the

diameter of its wire is 0.22 mm [57].

As shown in Fig. 6.20, a bending mechanism with a diameter of 15 mm has been

developed for gastrointestinal application. This mechanism consists of a stack of

elements with an SMA plate at the center of each element. When the SMA plates are

actuated by the application of current, the whole structure bends in two directions

like a vertebra of the human body [35].

Fig. 6.20 Active bending stack mechanism using SMA plate actuators [35] (Reprinted with

An active bending electric endoscope using SMA coil actuators with a CCD

camera has been developed (Fig. 6.21). The external diameter of the fabricated

endoscope is 5.5 mm, the external diameter of the utilized SMA coil actuator is

0.3 mm and the diameter of its wire is 0.075 mm [58].

6.3.1.3 Catheters

To inject contrast medium or medicine into the blood vessel or to measure blood

pressure locally, a catheter, which is a hollow ﬂexible small tube, is used. Catheter-

based procedures (catheterization) enable doctors to access almost every diseased

site which needs to be checked and treated via a blood vessel. Doctors control the tip

of the catheter by moving its proximal part from outside of the body. Depending on

blood vessel size, the catheters are approximately 0.3–3.0 mm in diameter and 1.5 m

in length. As doctors must control the tip of the catheter from outside the body, oper-

ations with catheters require considerable skill, particularly when the blood vessel

380 T. Mineta and Y. Haga

Fig. 6.21 Active bending

electric endoscope using

SMA coil actuators [58]

(Reprinted with permission.

of Electrical Engineers of

Japan)

has a loop or complex conﬁguration. Thus, there is a demand for an active catheter

which moves like a snake in the blood vessel. Precise and safe manipulation of the

catheter is realized if doctors can control the motion of the catheter from outside

the body. As back-and-forth operation of the catheter from outside the body is rela-

tively easy, a self-propelling mechanism is not necessarily demanded. Consequently,

almost all controllable steering mechanisms are installed near the tip of the catheter.

Active catheters which have micro-actuators at the tip have been developed for use

in every part of the body. The tip can be controlled from outside the body and moves

like a snake if many micro-actuators are distributed and properly controlled. Not

only bending motion, but also torsional and precise back-and-forth motion are useful

for manipulation of the tip of the catheter.

Plates or Sheets

An active catheter which has a bending mechanism realized by thin TiNi SMA plates

has been developed [59, 60]. As shown in Fig. 6.22, one of the SMA plates ﬁxed

along the side of the catheter bends when the plate is heated by application of a

current to restore its memorized shape. To realize precise control of bending motion

by feedback control using temperature monitoring, a component which has heaters,

temperature sensors and circuits on a ﬂexible polyimide ﬁlm substrate has been

fabricated using MEMS technology [60]. The ﬁlm components are ﬁxed on the TiNi

SMA plates placed on the sides of the catheter. To realize multi-directional bending,

pairs of ﬁlm components and SMA plates are ﬁxed serially on alternating sides of

the catheter. The external diameter of such a fabricated multi-joint catheter is 1 mm.

The SMA plates utilized for this catheter are two directionally memorized SMAs,

the shape of which is restored by cooling [60].

6 Materials and Processes in Shape Memory Alloy 381

Fig. 6.22 Micro-manipulator with SMA bendable ribbons attached with MIF (Thin ﬁlm heater

Wire

Shape memory alloy wires are also utilized for active catheters as shown in Fig. 6.23

[42, 61–63]. When the SMA wires are embedded in the catheter eccentrically and

heated above a certain transformation temperature by direct application of an elec-

trical current to the SMA wire, the wire actuator bends the catheter by contraction of

its length. The contraction length of the SMA wire, however, is relatively short and

SMA wires ﬁxed to the opposite side tend to restrict bending motion since the wires

have to bend and stretch passively. Consequently, it is difﬁcult to realize bending

with a small radius of curvature.

Coil

Shape memory alloy micro-coil actuators are used to obtain a large bending motion

and to realize multi-joint actuation as shown in Fig. 6.24. Shape memory alloy coils

enable multi-directional bending with a large bending angle because the SMA coil

actuators ﬁxed at the opposite side can be passively extended. Shape memory alloy

coil actuators can also be actuated by direct application of current when the wire

diameter of the coil is small. Three SMA coil actuators, which are extended (from

their memorized shapes) and ﬁxed in the catheter, contract and bend in several

directions. Multi-joint and multi-directional active catheters using silicon-glass link

structures have been fabricated. Three SMA coil actuators are ﬁxed between two

silicon-glass link structures at intervals of 120

◦

. Many joints are serially connected

and each joint can bend in any direction. The external diameter of the fabricated

catheter is 2.7 mm [44].

A major problem with active catheters which have many joints and func-

tions is the necessity of an excessive number of lead wires to control each

382 T. Mineta and Y. Haga

Fig. 6.23 Active bending catheter using SMA wires [62] (Reprinted with permission. Copyright

SMA actuator. To minimize the number of lead wires, ﬂexible polyimide-based

integrated complementary metal oxide semiconductor (CMOS) interface circuits

for communication and control (C&C) have been developed and utilized in

active catheters. To reduce the system size and simplify the assembly work,

the C&C integrated circuit (IC) and three lead wires are fabricated on the

same substrate using a CMOS-compatible polyimide-based process. The exter-

nal diameter of the fabricated catheter without an outer tube is approximately

2.0 mm [64].

By locating SMA coil actuators inside the liner coil and ﬁxing them to the liner

coil directly, link structures can be eliminated and the liner coil can be used as a

skeleton and a bias spring. Furthermore, every part works as a joint and hence bends

more ﬂexibly than that with links [65]. As the catheters for use in blood vessels

are disposable to avoid blood infection, active catheters need to be fabricated at

low cost. To solve this problem, batch assembly methods have been developed. One

is silicon wafer-level batch fabrication with a silicon joint structure [66] and the

other is fabrication using nickel electroplating and acrylic resin electrodeposition

6 Materials and Processes in Shape Memory Alloy 383

Fig. 6.24 Active bending catheter using SMA coils [44] (Reprinted with permission. Copyright

1996 Elsevier)

[67]. Fabrication using electroplating and deposition is carried out as follows. The

SMA coil and metal spring are coated with polymer and the polymer is partially

removed by laser ablation for partial exposure of the SMA surface and metal surface.

UV curable acrylic resin is locally deposited by electroplating between the exposed

SMA surface and the exposed metal surface. This novel method enables low cost

batch assembly and a small external diameter.

Instead of a coil shape, ﬂat meandering-shaped SMA actuators are also used

for active catheters (Fig. 6.25). The ﬂat shape meets both demands of small

external diameter and large working channel by realizing a thin wall [68]. A

batch fabrication method for ﬂat meandering SMA actuators from an SMA

(TiNi) sheet has been developed using electrochemical pulsed etching for active

catheters [40]. Furthermore, to simplify the assembly process of the SMA actua-

tors, three meandering actuators are formed in an SMA pipe using electrochemical

pulsed etching. The SMA pipe is dip-coated with photoresist, which is then

patterned using UV projection exposure and conventional development. This pro-

cess is more suitable for mass production than laser cutting and electrodischarge

machining.

Guide wires are wires which have a relatively ﬂexible tip and are used for

guidance of the catheter. An active bending guide wire has been fabricated using

a ﬂat meandering actuator (Fig. 6.26). The guide wire is bent by an electrical

current supplied to the actuator because the actuator has a memorized curved

shape [39].